Often, such motors must be capable of being operated on very differing voltages, e.g. a fan with a nominal voltage of 24 V is, in practice, operated with voltages ranging between 12 V and 32 V, which represents a voltage deviation range from -50% to +30%. Depending upon voltage, such a fan runs at a desired speed (RPM), i.e. slowly at lower voltages, and fast at higher voltages.
Upon commutation of the motor current from one stator winding phase to another, the switchover can occur in a "hard" or in a "soft" manner. A hard switchover offers good efficiency, but high structure-borne noise levels caused by commutation noise and, additionally, EMC problems (EMC=Electro-Magnetic Compatibility). Further, protective measures must be taken for the end-stage transistors, so that the critical limit values of the components (permissible collector voltages etc.) are not exceeded. This can be accomplished by internal Z-diodes contained in the end-stage transistors or by external Z-diodes (for limiting these voltages) or by recovery diodes which feed back the shutoff energy of the windings to an operating voltage link circuit containing a capacitor capable of receiving this shutoff energy.
FIG. 6 shows a motor 10 with two stator winding strands or phases 12, 14 and a (schematically indicated) permanent magnet rotor 16, in whose vicinity a Hall generator 18 is located. Hall generator 18 is also shown in the left portion of FIG. 6. Ohmic resistors of phases 12 and 14 are designated 20 and 22, respectively. Phase 12 is connected in series with an npn Darlington transistor 24 with built-in recovery diode 26, and phase 14 is connected in series with an npn Darlington transistor 28 with built-in recovery diode 30. The emitters of transistors 24, 28 are connected via a common emitter resistor 32 to a negative conductor 34. Phases 12, 14 are connected to a positive conductor 36, as shown. Conductors 34, 36 are, during operation, connected to a power supply device 38 which contains a storage capacitor 40. This serves to take up the shutoff energy of phases 12, 14, which is fed back via recovery diodes 26, 30 into this capacitor 40. To the extent that the motor is supplied from an accumulator, the shutoff energy is fed back into the accumulator.
Hall generator 18 is connected via a resistor 42 with positive conductor 36 and via a resistor 44 with negative conductor 34. Its output signal UH (FIG. 7A) is applied to both inputs IN1 and IN2 of an IC (Integrated Circuit) 46 which generates signals OUT1 and OUT2 for controlling transistors 24, 26 and simultaneously serves as blocking or stall protection for motor 10, i.e. when it is unable to turn, it is switched off by IC 46.
This IC is preferably the ROHM BA6406. FIG. 7A shows the signal UH, FIG. 7B shows the signal OUT1 of IC 46 and FIG. 7C shows the signal OUT2. Signals OUT1 and OUT2 run in phase opposition to each other. FIG. OUT1 is fed via a resistor 50 (e.g. 8.2 k.OMEGA.) to the base of transistor 24, which is connected via a capacitor 52 (e.g. 1 Nf) to the collector and via a base drain-off resistor 54 (e.g. 1.2 k.OMEGA.) to negative conductor 34. In the same manner, signal OUT2 is fed via a resistor 56 to the base of transistor 28, which is connected via a capacitor 58 with its collector and via a base drain-off resistor 60 with negative conductor 34.
During operation, transistors 24, 28 are alternately switched on by signals OUT1, OUT2. Through the combination of resistor 50 and capacitor 52 and the common resistor 32, a soft switching of transistor 24 is achieved. Similarly, resistor 56, in combination with capacitor 58 and common resistor 32, effects a soft switching of transistor 28. However, these measures cause an increased warming of transistors 24, 28 and therefore a reduction in efficiency. Furthermore, the circuit can be optimally configured only for a single operating point, i.e. for a specified RPM and a specified torque. This results in many compromises, especially with respect to voltage overruns and temperature overruns. One also obtains, in practice, a voltage range only of .+-.15%.